Plasminogen activators (PA) in the euglobulin fraction of dextran sulfate activated plasma (DS-EF) were assayed on fibrin plates. Activity related to tissue plasminogen activator (t-PA) or urokinase (u-PA) was quantified by antiserum inhibition. The DS-EF contained 30% t-PA, 30% u-PA and 40-50% activity unrelated to t-PA or u-PA. The latter was completely inhibited by 1.7 mumol/1 C1-inhibitor (C1INH), the two former were less sensitive. Addition of flufenamate to the DS-EF (DS-EF/Fluf) from normal and two factor XII (F XII)-deficient plasmas increased their activities to the same high level. More than 50% of the activity was unrelated to t-PA or u-PA, 30-40% was u-PA and 5-10% t-PA related. After addition of fibrinogen to DS-EF/Fluf and clotting with thrombin, the remaining solution contained only about 30% of the total activity, including less than 10% u-PA. The epsilon-aminocaproic acid inhibition pattern obtained with the DS-EF was uniform, and thus different from the biphasic pattern obtained with the low fibrin affinity PA, two-chain urokinase. Thus, both the plasma u-PA and the major unidentified PA in plasma have affinity for fibrin.